Field of the Invention
The present invention relates to a light emitting diode, and more specifically to a white light emitting diode including a p-type semiconductor layer, a conductivity type reflective layer formed on the p-type semiconductor layer to reflect light of a short wavelength band propagating through the p-type semiconductor layer, and a metal electrode formed on the conductivity type semiconductor layer to further reflect light of a short wavelength band and light of a visible wavelength band.
Description of the Related Art
Generally, nitrides of Group III elements, such as gallium nitride (GaN) and aluminum nitride (AlN), have recently received much attention as materials for visible and ultraviolet light emitting devices due to their good thermal stability and direct transition type energy band structure. Particularly, blue and green light emitting devices using indium gallium nitride (InGaN) are utilized in various applications, including large-area full-color flat panel displays, traffic signal lamps, indoor lighting systems, high-density light sources, high-resolution output systems, and optical communications.
It is difficult to grow a nitride semiconductor layer of a Group III element on a substrate made of the same material. Due to this difficulty, a nitride semiconductor layer of a Group III element is grown on a substrate made of a dissimilar material with a similar crystal structure by a suitable process, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). A sapphire substrate having a hexagonal system structure is mainly used as the dissimilar substrate. However, sapphire is an electrical insulator, limiting the structure of a light emitting diode. A technique for fabricating a vertical light emitting diode with high efficiency has been recently developed in which epitaxial layers, such as nitride semiconductor layers, are allowed to grow on a growth substrate made of a dissimilar material, such as sapphire, a support substrate is bonded to the epitaxial layers, and the growth substrate is separated by a suitable process, such as a laser lift-off process. According to this technique, a vertical light emitting diode is fabricated by sequentially forming an n-type GaN layer, an active layer, and a p-type GaN layer on a sapphire substrate as a growth substrate, forming a p-type ohmic electrode or an ohmic reflective layer on the p-type GaN layer, bonding a support substrate thereon, removing the sapphire substrate, and forming an electrode pad on the exposed n-type compound semiconductor layer.
On the other hand, a flip-chip light emitting diode has been developed that achieves high luminance and high output without the need for bonding wires to the sides of an electrode pad. The flip-chip light emitting diode is generally constructed such that light is emitted through a sapphire substrate rather than through a p-type semiconductor layer. This construction enables the use of a thick p-type electrode, ensuring good current spreading in the p-type semiconductor layer. In addition, heat can be dissipated through the sub mount substrate, resulting in a significant reduction in thermal resistance.
In the flip-chip light emitting diode and the vertical light emitting diode, light emitted from the active layer to the p-type semiconductor layer should be reflected towards the substrate. The formation of a reflective layer is necessary for the reflection of light emitted to the p-type semiconductor layer. A metal electrode is generally used to perform the function of the reflective layer.